SAN FRANCISCO, April 19 (Reuters) - Lyft Inc is launching a program on Thursday to offset emissions from the 1.4 million cars that drive for the U.S. ride-hailing service by investing in projects to reduce other sources of greenhouse gas, the company said.

The move comes as Lyft and Uber Technologies Inc, as well as bike-share and scooter-renting startups, face scrutiny for clogging city streets and sidewalks, worsening traffic.

The company will purchase carbon "offsets" to support projects such as renewable energy and reforestation, balancing the carbon impact of every mile driven by a car working for Lyft, including the miles driven to pick up a passenger. That will be a multi-million dollar investment in the first year and the cost will rise as Lyft grows.

"This in a sense puts a tax on ourselves to continue to move toward shared rides and lower emission vehicles," said Lyft Co-founder and President John Zimmer in an interview.

The company declined to provide a precise dollar amount for the program. Lyft drivers completed 376 million rides in 2017, accounting for well over a billion miles, according to the company.

Lyft said in a statement it is working with environmental firm 3Degrees, which sources carbon reduction projects. Lyft will tap its more than $4 billion in venture capital funding to pay for the carbon offsets.

Zimmer said the project took on a heightened sense of urgency since the U.S. Environmental Protection Agency this month moved to ease Obama-era vehicle emissions standards.

Initial carbon-offset projects funded by Lyft include an effort to reduce greenhouse gas emission from an auto parts factory in Michigan by using alternative materials and recycling transformer oil in Ohio.

"It's really important that we know what the projects are, that they are real and tangible and that they would not have happened otherwise," Zimmer said.

Lyft's carbon-neutral effort is the latest example of a Silicon Valley company working to mitigate some of the harm its services have caused. This week, Airbnb announced its Office of Healthy Tourism, aimed at easing crowding from tourism that has plagued cities such as Barcelona and Amsterdam, a problem exacerbated by Airbnb.

San Francisco's city attorney last year issued subpoenas to Lyft and Uber for records of driving practices amid concerns the companies had created traffic hazards. Zimmer acknowledged "traffic is a major issue" and said the company also has plans to invest in electric autonomous vehicles and expand its carpooling service. (Reporting by Heather Somerville; Editing by Cynthia Osterman)